Caption: Calabar beans. Calabar beans (Physostigma venenosum) in their pods. These beans are the source of the drug physostigmine. This is used in the treatment of glaucoma, where a preparation is dropped directly onto the eyeball. It also acts as a muscle relaxant, and has been used to treat epilepsy, tetanus and constipation. Overdoses of physostigmine can be fatal.